The present invention relates generally to the imaging of objects in turbid media and more particularly to a novel technique for imaging objects in turbid media.
As can readily be appreciated, there are many situations in which the detection of an object present in a turbid, i.e., highly scattering, medium is highly desirable. For instance, the detection of a tumor embedded within a tissue is one such example. Although X-ray techniques do provide some measure of success in detecting objects in turbid media, they are not well-suited for detecting very small objects, e.g., tumors less than 1 mm in size, or for detecting objects in thick media. In addition. X-ray radiation can present safety hazards to a person exposed thereto.
An alternative technique used to detect objects in turbid media is transillumination. In transillumination, visible light is incident on one side of a medium and the light emergent from the opposite side of the medium is used to form an image. Objects embedded in the medium typically absorb the incident light and appear in the image as shadows. Unfortunately, the usefulness of transillumination as a detection technique is severely limited in those instances in which the medium is thick or the object is very small. This is because light scattering within the medium contributes to noise and reduces the intensity of the unscattered light used to form the image shadow.
To improve the detectability of small objects located in a turbid medium using transillumination, many investigators have attempted to selectively use only certain components of the transilluminating light signal. This may be done by exploiting the properties of photon migration through a scattering medium. Photons migrating through a turbid medium have traditionally been categorized into three major signal components: (1) the ballistic (coherent) photons which arrive first by traveling over the shortest, most direct path; (2) the snake (quasi-coherent) photons which arrive within the first .delta.t after the ballistic photons and which deviate, only to a very slight extent, off a straight-line propagation path; and (3) the diffusive (incoherent) photons which experience comparatively more scattering than do ballistic and snake photons and, therefore, deviate more considerably from the straight-line propagation path followed by ballistic and snake photons.
Because it has been believed that ballistic and snake photons contain the least distorted image information and that diffusive photons lose most of the image information, efforts to make transillumination work most effectively with turbid media have focused on techniques which permit the selective detection of ballistic and snake photons while rejecting diffusive photons. This process of selection and rejection have been implemented in various time-gating, space-gating and time/space-gating techniques. Patents, patent applications and publications which disclose certain of these techniques include U.S. Pat. No. 5,140,463, inventors Yoo et al., which issued Aug. 18, 1992; U.S. Pat. No. 5,142,372, inventors Alfano et al., which issued Aug. 25, 1992; U.S. Pat. No. 5,227,912, inventors Ho et al., which issued Jul. 13, 1993; U.S. Pat. No. 5,371,368, inventors Alfano et al., filed Jul. 23, 1992; Alfano et al., "Photons for prompt tumor detection," Physics World, pp. 37-40 (January 1992); Wang et al., "Ballistic 2-D Imaging Through Scattering Walls Using an Ultrafast Optical Kerr Gate," Science, Vol. 253, pp. 769-771 (Aug. 16, 1991); Wang et al., "Kerr-Fourier imaging of hidden objects in thick turbid media," Optics Letters. Vol. 18, No. 3, pp. 241-243 (Feb. 1, 1993); Yoo et al., "Time-resolved coherent and incoherent components of forward light scattering in random media," Optics Letter, Vol. 15, No. 6, pp. 320-322 (Mar. 15, 1990); Chen et al., "Two-dimensional imaging through diffusing media using 150-fs gated electronic holography techniques," Optics Letters, Vol. 16, No. 7, pp. 487-489 (Apr. 1, 1991); Duncan et al., "Time-gated imaging through scattering media using stimulated Raman amplification." Optics Letters, Vol. 16, No. 23, pp. 1868-1870 (Dec. 1, 1991), all of which are incorporated herein by reference.
Of the above-listed art, Wang et al., "Kerr-Fourier imaging of hidden objects in thick turbid media." Optics Letters, Vol. 18, No. 3, pp. 241-243 (Feb. 1, 1993) is illustrative. In this article, there is disclosed a time/space-gating system for use in imaging opaque test bars hidden inside a 5.5 cm-thick 2.5% Intralipid solution. The disclosed system includes three main parts: a laser source, an optical Kerr gate and a detector. The laser source is a picosecond mode-locked laser system, which emits a 1054 nm, 8 ps laser pulse train as the illumination source. The second harmonic of the pulse train, which is generated by transmission through a potassium dihydrate phosphate (KDP) crystal, is used as the gating source. The illumination source is sent through a variable time-delay and is then used to transilluminate, from one side, the turbid medium containing the opaque object. The signal from the turbid medium located at the front focal plane of a lens is collected and transformed to a Kerr cell located at its back focal plane (i.e., the Fourier-transform spectral plane of a 4F system). That portion of the Kerr cell located at the focal point of the 4F system is gated at the appropriate time using the gating source so that only the ballistic and snake components are permitted to pass therethrough. The spatial-filtered and temporal-segmented signal is then imaged by a second lens onto a CCD camera.
Although techniques of the type described above, which selectively use ballistic and snake photons to image objects in turbid media, have enjoyed a modicum of success, such techniques have been limited by the fact that detected light signals derived from ballistic and snake photons are typically rather weak, due to the proportionately small number of transilluminated ballistic and snake photons. This problem is further exacerbated in those instances in which the turbid medium is thick and the likelihood of substantial scattering increases.
An experimental imaging scheme using diffusive light requires a theoretical algorithm to restore the internal properties of an object from the array of measured experimental data. The necessity for using mathematical methods is evoked by the fact that measured quantities are merely integral characteristics of the object properties and that the diffusive photons have lost their potential for shadowgram since they travel over the medium in random ways. The diffusive photons wander over the medium in a tortuous manner. These photons sample various parts of the medium, some never entering the defect.
Over the years, there have been several efforts to use diffusive light to image inside and map the internal structure of an object. These efforts depend on how one inverts the experimental scattering data obtained around the object to give information on the various points in the medium using some inverse algorithm and reconstruction approach. The equation used mainly depends on the diffusion equation, higher approximation or just the probability of the random walk given by the Monte Carlo method. Since the photons wander about, this problem is ill posed. There are many attempts at solving the problem and the path undergone. There are a number of models based on the diffusion approximation (e.g., M. Patterson et al., SPIE, 1767, 372 (1992); J. Schotland et al., App. Opt., 32, 448 (1993)). Though the latter uses the concept of photon paths, it remains completely in frames of standard diffusion approximation. The algorithms based upon the diffusion approximation would not lead to a resolution that is better than 5-10 mm. These techniques use absorption changes in tissue to determine the inverse from the transmission data inverse algorithm. The diffusion approximation is not valid at a distance smaller than 7 l.sub.tr where l.sub.tr is 1 to 2 mm giving 7 to 14 mm resolution. In addition, these approaches need referenced medium to compare with to determine the differences. Furthermore, the absorption in tissue in the near IR is low, absorption length l.sub.a .gtoreq.50 mm which means that mm detects will not be simply seen or detected. The absorption length l.sub.a =size of entire object. Detects will not be observed using absoption changes. J. Singer and F. Grunbaum et al., Science, 248, 990 (1990); Grunbaum, SPIE, 203, 1887 (1993) proposed an approach based on simulating photon migration as a random walk on a two (three) dimensional grid of pixels (voxels). Each pixel (voxel) k is characterized by a set of parameters {w}.sub.k describing its absorption and scattering efficiency. The probability P.sub.ij for a photon launched at a pixel i on the boundary of a medium to be detected at the pixel j can be calculated. Considering P.sub.ij as experimental data the set {w}.sub.k can be restored. So far only the two-dimensional case has been numerically realized with photons allowed to wander in four or eight directions in an isotropic manner.
A linear perturbation approach was used by R. Barbour et al., SPIE, 192, 1437 (1991), OSA Proceeding on Advances in Optical Imaging and Photon Migration, V21, 211 (1994), in order to overcome intractable difficulties in solving the inverse problem. They assume that the absorption properties of a medium with unknown defects (X.sub.i) are very close to that of a reference medium (X.sub.i.sup.r), X.sub.i.sup.r being the absorption efficiency of the volume element i. This method depends on prior knowledge of absorption properties of the reference media and linear response function (matrix W) connecting changes in absorption properties object .DELTA.X caused by multiple defects with changes in the array of measured signals .DELTA.I around the object: .DELTA.I=W.DELTA.X. Given W and .DELTA.I from the measurement, .DELTA.X can be evaluated. Then X can be found. The matrix W was precalculated using the Monte Carlo approach in simulating photon migration in the reference medium. These calculations should be done for each particular shape of an object and a source-detector arrangement. So far, the simplest rectangular shape of a medium was considered and a reference turbid medium was assumed to be uniform.
Several standard numerical algorithms to minimize quadratic functionals including Tikhonov regularization approach to stabilized minimization procedure were used to find approximate solution of the linear problem W.DELTA.X=.DELTA.I. However, when the reference medium is very different from the test medium, the linear approximation is inaccurate which is the case in most real medical applications, say, breast and brain imaging.
There are too many shortcomings of the above methods, besides the mentioned ones: 1) they are ignoring a specific feature of biological tissues that scattered light is highly forward directed, 2) detectors are assumed to collect together photons arriving in different directions. This means that detection technique and the respective algorithms introduce additional ill-posedness to the imaging problem.
There is a need for a reconstruction method from scattered temporal data about the object which does not depend on the absorption or reference medium to compare with, which does not strictly depend on the diffusive equation, and which exploits more real physical features of the object.